For example, in a steam generator of a pressurized water reactor (PWR), primary cooling water in a water chamber is evacuated at the time of maintenance or the like. At this time, since the channel head forming the water chamber is formed in a bowl shape, in some cases, the primary cooling water remains in the bottom thereof or the primary cooling water remains in a lid of a manhole. Thus, the channel head is formed with a drain hole leading to the bottom side of the channel head from the manhole, and a drain hole leading to a nozzle from the bottom of the channel head (for example, see Patent Literature 1). The drain hole is configured so that a drain tube made of a corrosion-resistant material is inserted and mounted to the through hole formed in the channel head in consideration of corrosion resistance.